In an automatic gain control circuit, the output level from a variable gain amplifier is maintained constant by comparing the output level from the variable gain amplifier with a reference signal, and controlling the gain of the variable gain amplifier so that the output level from the variable gain amplifier matches a reference value.
In the automatic gain control circuit disclosed in Japanese Patent Application Publication No. 11-195941, a band-pass filtering process capable of sufficiently suppressing the interference of adjacent channels is performed on the reception signal subjected to orthogonal detection, a desired wave power, and an interfering wave power are obtained, and a loop filter coefficient is calculated with reference to such power values to perform the gain control that follows the desired wave power fluctuation without following the undesired wave power.
However, in the automatic gain control circuit, the response time until the gain of the variable gain amplifier converges increases if the amplitude of the input signal is small since the gain of the variable gain amplifier is controlled by an integrating operation of the loop filter.
In the automatic gain control circuit disclosed in Japanese Patent Application Publication No. 11-195941, the response time until the gain of the variable gain amplifier converges cannot be reduced since the loop gain of the loop filter is controlled based on the magnitude of the desired wave power and the undesired wave power.
The input/output characteristics of the automatic gain control circuit are generally known to be a linear response in the logarithmic region. Thus, the response time until the gain of the variable gain amplifier converges can be reduced by linear/logarithmic converting the output level from the variable gain amplifier, but the scale of the circuit becomes large when realizing the linear/logarithmic conversion through digital signal processing.